Cell surface glycans are major determinants of cell-cell and cell-matrix interactions. Their structures reflect the expression of glycosyltransferases and sulfotransferases that act in an assembly line within the Golgi compartment. The broad objective of this project is to develop chemical tools for studying Golgi enzymes and the functions of the glycans they produce on cells. The last granting period focused on a newly discovered family of GlcNAc-6-sulfotransferases. The goals were to (1) develop small molecule inhibitors of the sulfotransferases as tools for biological studies and leads for drug discovery, (2) identify residues involved in substrate binding and catalysis, and (3) determine the preferred cellular substrates as a step toward elucidating biological function. An exciting discovery from this work was that the substrate preference of each enzyme in vivo is governed largely by its distribution among the Golgi cisternae. Thus, Golgi localization was identified as a major determinant of biological function. The Specific Aims of the next granting period build from this discovery. The major objective of the next granting period is to develop an approach for modulating Golgi enzyme activity with small molecules that target their common requirement of Golgi localization. The proposed strategy is based on the chemical dimerizer-induced assembly of the enzymes' modular catalytic and localization domains. The approach was validated with fucosyltransferase 7 (FucT7) and the GlcNAc-6- sulfotransferases GST-2 and GST-3, using the rapamycin/FRB/FKBP system for inducible domain assembly. The Specific Aims of the next granting period expand upon this discovery in three directions. The first Aim is to investigate the FucT7 system in more detail in order to define those parameters that affect the cellular activity of the reconstituted domains. The goal is to optimize the current FucT7 system for application to studies of tumor cell metastasis and for use in transgenic mice. The second Aim is to apply the approach to other Golgi enzymes, chosen for their diversity of substrates and functions. The third and final Aim is to determine whether chemical dimerizers can be used to modulate associations between two glycosyltransferases. Such associations are thought to be important for the efficiency of glycolipid biosynthetic pathways. The ability to modulate glycosyltransferase associations will provide means to control glycolipid expression on cells. [unreadable] [unreadable]